How to check if a term is parent to another?

You can use wp_list_categories() where the hierarchical argument is set to true by default , something like this:

<div class="produkt_nav"><h3><a href="https://wordpress.stackexchange.com/produkter/">Produkter</a></h3>
    <ul>
    <?php wp_list_categories('taxonomy' => 'kategori'); ?> 
    </ul>
</div>

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